[majormajor]

Except for a year in a dorm where I couldn't disable my dorm room door's automatically-lock-when-shut behavior, I've developed a simple habit that's kept me from ever locking myself out of my car or home since: never lock the place/vehicle from the inside, or let it lock itself. I can't lock the deadbolt on my apartment when I leave without the key. And I can't lock my car from outside without the key. It's a way of forcing me to always have my keys.

So while I love the convenience this idea provides for shared access to places, and for frictionless don't-even-have-to-get-out-my-keys access, locking oneself out is a solved problem in my book. (Sure, I could lose my keys, but I could also lose my phone—and the redundancy here is another nice and fun thing provided by this system.)

(This habit was largely inspired by getting a car which refused to let me lock myself out: if I pushed the lock button down on the driver's door while the door was open, and then shut the door, it unlocked itself. Previously I'd had one where you held the handle out to override this behavior, but this one insisted on being locked from the outside. And then I noticed that this was keeping my from locking myself out like I had a few times with the old one.)

[anonymouz]

The automatically-lock-when-shut behavior of doors is an extremely weak protection anyway, so one should always lock the door with a key anyway (this at least prevents the most trivial circumvention, i.e., open the door with a plastic card).

Interestingly I seem to use more or less the converse of your method, also with great success: Always lock the apartment door from the inside. Whenever I leave the apartment, I therefore am forced to take the keys.

I suppose the most important thing is to be consistent with ones behavior, so that a subconscious habit can develop.